HARRISON, N.Y. -- Harrison High School has been named the fourth-most challenging high school in Westchester County and the 18th-most challenging high school in New York State, according to a new national study of public and private high schools conducted by the Washington Post.

According to the report, the study ranks schools through an index formula that is a simple ratio: the number of Advanced Placement, International Baccalaureate and Advanced International Certificate of Education tests given at a school each year, divided by the number of seniors who graduated that year. A ratio of 1.000 means the school had as many tests as graduates.

Also noted are the percentage of students who come from families that qualify for lunch subsidies and the percentage of graduates who passed at least one college-level test during their high school career, called equity and excellence, according to the report.

Harrison High School received an index score of 4.205 with its 94-percent four-year graduation rate, average SAT score of 1,577, average ACT score of 23.7 and an offering of six Advanced Placement courses and 24 International Baccalaureate courses.

Harrison also was ranked the 22nd-most challenging school in the Northeast region and 217th nationally.

City Honors in Buffalo was ranked the most challenging school in New York State with an index of 9.121. American Indian Public Charter School in Oakland, Calif., was named the most challenging school in America with an index of 21.909 in the Post's study.

The top 10 most-challenging high schools in Westchester County, according to the study are: